/*
 * Copyright (c) 2013, SRI International
 * All rights reserved.
 * Licensed under the The BSD 3-Clause License;
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 * 
 * http://opensource.org/licenses/BSD-3-Clause
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 
 * Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 * 
 * Neither the name of the aic-expresso nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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package com.sri.ai.grinder.library.equality.formula.helper;

import java.util.ArrayList;
import java.util.List;

import com.google.common.annotations.Beta;
import com.sri.ai.expresso.api.Expression;
import com.sri.ai.expresso.helper.Expressions;
import com.sri.ai.grinder.api.RewritingProcess;
import com.sri.ai.grinder.core.AbstractRewriter;
import com.sri.ai.grinder.core.HasKind;
import com.sri.ai.grinder.library.Equality;
import com.sri.ai.grinder.library.FunctorConstants;
import com.sri.ai.grinder.library.boole.And;

/**
 * Performs the following normalizations on formulas:
 * 
 * X = Y = Z = -> X = Y and Y = Z
 * a = X       -> X = a
 * B = A       -> A = B
 * X = X       -> true
 * a = b       -> false
 */
@Beta
public class NormalizeEqualityLiteral extends AbstractRewriter {
	
	public NormalizeEqualityLiteral() {
		this.setReifiedTests(new HasKind(FunctorConstants.EQUAL));
	}
	
	
	@Override
	public Expression rewriteAfterBookkeeping(Expression expression,
			RewritingProcess process) {
		Expression result = expression;			

		// X = Y = Z -> X = Y and Y = Z
		if (expression.numberOfArguments() > 2) {
			List<Expression> conjuncts = new ArrayList<Expression>();
			for (int i = 0; i < expression.numberOfArguments()-1; i++) {
				conjuncts.add(Equality.make(expression.get(i), expression.get(i+1)));
			}
			result = And.make(conjuncts);
		}
		else {
			// a = X  -> X = a
			Expression normalized = Equality.normalize(expression, process);
			if (normalized != expression) {
				result = normalized;
			}
			else {
				// B = A  -> A = B
				String e0 = expression.get(0).toString();
				String e1 = expression.get(1).toString();
			    if (e0.compareTo(e1) > 0 && process.isVariable(expression.get(0)) && process.isVariable(expression.get(1))) {
			    	result = Expressions.makeExpressionOnSyntaxTreeWithLabelAndSubTrees(expression.getFunctor(), expression.get(1), expression.get(0));
			    }
				// X = X  -> true
			    else if (expression.get(0).equals(expression.get(1))) {
					result = Expressions.TRUE;		
				}
				else {
					// a = b  -> false
					if (process.isUniquelyNamedConstant(expression.get(0)) && process.isUniquelyNamedConstant(expression.get(1))) {
						result = Expressions.FALSE;
					}
				}
			}
		}

		return result;
	}
}
